Ill 


Intelligence  of  Troops  Infected  with  Hookworm 
vs.  Those  not  Infected 


BY 
GARRY  C.  MYERS 

Head  of  Department  of  Psychology,  School  of  Education,  Cleveland,  Ohio 


Reprinted  from  THE  PEDAGOGICAL  SEMINARY 
October,  1920,  Vol.  XXVII,  pp.  211-242 


B10UC 
UBR" 


INTELLIGENCE     OF     TROOPS     INFECTED     WITH 
HOOKWORM  VS.  THOSE  NOT  INFECTED 


By  GARRY  C.  MYERS 
Head  of  Dept.  of  Psychology,  School  of  Education,  Cleveland,  O. 


HISTORY  AND  PROCEDURE 

During  the  War,  Major  B.  F.  Pittenger,  Sanitary  Corps, 
chief  psychological  examiner  at  Camp  Sevier,  in  collaboration 
with  Major  Charles  A.  Kofoid,  Sanitary  Corps,  in  charge  of 
the  Laboratory  Car  Metchnikoff,  made  a  study  of  the  com- 
parative intelligence  ratings  of  the  recruits  found  to  have 
hookworm  infection  and  those  without  infection,  of  the  9,254 
men  of  the  April,  1918,  draft  increment.  A  report  thereon 
to  the  Office  of  the  Surgeon  General,  Division  of  Psychology, 
was  made  by  Major  Pittenger  in  comparative  tables  for  hook- 
worm and  non-hookworm  groups,  respectively,  in  terms  of 
Alpha  and  Beta  scores,  for  white  and  for  colored  troops.  Of 
these  data  Major  Charles  A.  Kofoid  and  Lieutenant  J.  P. 
Tucker,  Medical  Corps,  offer  some  interpretation  in  their  re- 
port to  the  Laboratory  Division  of  the  Surgeon  General's 
Office  "  on  the  relationship  of  infection  by  hookworm  to  the 
incidence  of  morbidity  in  22,842  men  in  the  United  States 
Army  at  Camp  Bowie,  Texas,  from  October,  1917,  to  April, 
1918." 

The  manifest  inferiority  of  the  median  intelligence  scores 
of  the  762  hookworm  cases  to  the  8,492  non-hookworm  cases 
suggested  the  desirability  of  a  more  comprehensive  study 
wherein  each  hookworm  case  could  be  paired  with  a  non- 
hookworm  case  from  the  same  local  community.  Accordingly 
Major  Kofoid  recommended  to  the  Surgeon  General  a  co- 


43C2S2. 


212  fyt^ftljCfeNt'E'OF  JTRpQPS  INFECTED  WITH  HOOKWORM 

operative  study  of  the  problem  by  the  Laboratory  Division 
and  the  Section  of  Psychology.  A  psychological  officer  was 
not  immediately  available,  but  Major  John  L.  Riley,  Sanitary 
Corps,  was  released  by  the  Section  of  Physical  Reconstruction 
for  temporary  duty  with  Major  Kofoid  in  organizing  and 
promoting  the  early  work  of  this  investigation.  Under  their 
direction  a  group  of  officers,  laboratory  technicians  and  en- 
listed men  proceeded  to  assemble  and  to  code  for  Hollerith 
treatment  the  intelligence  ratings  of  recruits  infected  with 
hookworm  and  of  their  non-infected  pairs. 

During  the  second  week  in  July,  1919,  Captain  Garry  C. 
Myers  of  the  psychological  personnel  became  available  for 
duty  on  this  investigation.  He  reported  to  the  Surgeon  Gen- 
eral to  select  and  ship  records  from  the  files  of  the  Section  of 
Psychology  to  the  Port  of  Embarkation,  where  initial  work 
was  already  in  progress.  A  week  later  he  was  ordered  to 
U.  S.  A.  Laboratory,  Port  of  Embarkation,  New  York  City, 
for  duty  in  connection  with  the  statistical  treatment  of  the 
records. 

The  personnel  at  the  laboratory,  in  charge  of  Lieut.  Col. 
Edwin  H.  Schorer,  varied  from  seventeen  officers,  enlisted 
men  and  women  during  July  and  early  August  to  five  officers 
and  assistants  in  early  September.  Different  officers  from 
time  to  time  awaiting  their  discharge  were  assigned  for  but  a 
few  days,  whereas  Second  Lieutenant  Conrad  Erwin  Ronnen- 
berg,  Sanitary  Corps,  and  Lieutenant  Henry  S.  Weigle,  Medi- 
cal Corps,  were  indispensable  collaborators  throughout  the 
whole  period  of  the  work  at  the  port  of  embarkation. 

Major  Kofoid  left  the  service  on  August  8th  and  Major 
Riley  left  the  service  on  September  6th,  leaving  the  work  in 
charge  of  Captain  Myers,  who  again  reported  at  the  office  of 
the  Surgeon  General  on  September  10th  to  complete  the  study. 
Upon  his  arrival,  it  was  found  that  inordinate  demands  upon 
the  statistical  department  eliminated  all  hope  of  final  Hollerith 
tabulation  of  the  data  and  it  appeared  that  completion  of  the 
study  would  have  to  be  postponed  indefinitely.  However, 
through  the  interest  and  initiative  of  Colonel  Joseph  F.  Siler, 
Medical  Corps,  of  the  Laboratory  Division  and  Colonel  F.  F. 
Russell,  Medical  Corps,  of  the  Army  Medical  School,  two 
enlisted  men  were  assigned  to  assist  Captain  Myers  in  assem- 
bling the  data  by  the  traditional  method  of  hand  tallying. 
Frequency  tables  were  thus  completed  in  about  three  weeks. 
Sergeant  Benjamin  M.  Oppenheim  was  then  loaned  by  the 
Division  of  Coordination,  Organization  and  Equipment  to  plot 
the  graphs. 

Thus  it  will  be  perceived  that  Major  Kofoid  initiated  the 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  213 

study  and  the  larger  machinery  for  its  execution;  Major  Riley 
supervised  the  greater  portion  of  the  work  assembling  the 
data  for  this  study  at  the  port  embarkation;  and  Captain 
Myers  is  responsible  for  the  final  assembling  and  treatment 
of  data  with  their  interpretations. 

The  methods  of  pulling  intelligence  rating  cards  was  as  fol- 
lows :  Each  time  a  card  of  a  hookworm  soldier  was  found, 
another  card  was  paired  with  it,  of  a  soldier,  reported  on 
hookworm  rosters  as  not  having  hookworm,  and  whose  home 
address  was  of  the  same  county  as  that  of  the  hookworm  case 
with  whose  card  it  was  paired. 

The  study  was  started  with  the  hope  to  compare  10,000 
hookworm  cases  paired  with  10,000  non-hookworm  cases  from 
the  same  local  areas  in  terms  of  home  addresses.  However, 
owing  to  the  frequent  disparity  between  the  groups  and  organ- 
izations surveyed  for  hookworm  and  the  groups  and  organiza- 
tions surveyed  by  the  intelligence  rating  boards  (since  the 
latter  boards  were  organized  in  some  camps  subsequent  to  the 
earlier  hookworm  surveys)  and  to  the  absence  of  necessary 
data  either  from  the  hookworm  rosters  or  from  the  intelli- 
gence rating  cards,  only  6,639  hookworm  and  6,639  non-hook- 
worm cases  were  studied,  of  which  number  of  each  there  were 
612  cases  of  colored  troops. 

In  every  instance  where  two  or  more  examinations  had  been 
given,  that  score  was  counted  which  on  the  basis  of  the  equiva- 
lent mental  age  scale  as  per  the  Psychological  Examiner's 
Guide,  page  91,  gave  the  highest  record.  All  Alpha  and  all 
Beta  records  were  assembled  in  separate  tables  and  all  indi- 
vidual-tests records  were  merged  with  Alpha  and  Beta  scores 
into  a  common  scale  on  the  basis  of  equivalent  mental  ages, 
which  in  turn  were  converted  into  equivalent  letter  ratings  as 
per  Examiner's  Guide,  page  91. 

The  statisticians1  in  the  Section  of  Psychology  of  the  Sur- 
geon General's  Office  who  had  developed  these  mental  age 
and  letter  rating  equivalents  to  meet  an  urgent  need  during 
the  war,  had  found  in  a  later  study  that  these  equivalents  were 
but  roughly  accurate.  Consequently  they,  with  the  help  of 
Karl  Pearson,  developed  by  an  elaborate  and  exhaustive  study 
the  groundwork  of  very  refined  equivalents.  However,  these 
statisticians  left  the  service  before  a  table  was  developed 
applicable  for  this  study.  Certainly  then  the  ratings  of  this 
study,  which  are  indicated  by  the  "  Common  Scale,"  are  not 
so  exact  as  they  ought  to  be ;  yet  they  are  the  best  which  the 
available  standards  offered  as  expedient. 

1  Mr.  Carl  R.  Brown  and  Lt.  Mark  A.  May. 

m0s?&  )  ^Yrl' 

•' 


214  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 

In  addition  to  the  intelligence  rating  the  chronological  age 
and  years  of  schooling  were  taken  into  account.  Rank  also 
was  noted  early  in  the  study  but  because  the  intelligence  record 
cards — as  would  be  expected  since  they  were  taken  soon  after 
the  recruits  entered  camp — listed  practically  all  soldiers  as 
privates,  rank  difference  proved  sterile  for  this  study. 

Furthermore,  the  record  cards  were  pulled  on  the  basis  of 
"  heavy  territory  "  and  "  light  territory,"  the  former  meaning 
a  county  in  which  the  army  hookworm  survey  indicated  more 
than  10%  of  recruits  infected  with  hookworm;  the  latter,  less 
than  10%  of  recruits  infected.  In  case,  then,  an  intelligence 
record  card  was  found  of  a  soldier  infected  with  hookworm, 
whose  home  address  was  in  a  light  county,  the  intelligence 
record  card  of  a  non-hookworm  soldier  from  the  same  county 
was  pulled. 

With  the  negroes,  whose  intelligence  rating  cards  as  well 
as  hookworm  survey  rosters  were  sometimes  designated  as 
merely  from  Texas,  say,  or  Tennessee,  it  was  almost  impos- 
sible to  compare  light  territory  and  heavy  territory  groups. 
This  fact,  in  addition  to  the  relatively  small  number  of  cases, 
explains  why  the  colored  troops  are  limited  to  one  general 
table  of  comparison  of  hookworm  cases  with  non-hookworm 
cases. 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  215 

RESULTS 
TABLE  I 

PER  CENT  DISTRIBUTION  OF  RATINGS 
All  White  Troops 


Alpha 

Beta 

Com.  Scale 

Hook- 

Non- 

Hook- 

Non- 

Hook- 

Non- 

worm 

hook- 

worm 

hook- 

worm 

hook- 

worm 

worm 

worm 

A 

1.2 

3.0 

0.7 

1.1 

1.5 

3.5 

B 

4.4 

7.9 

1.1 

2.2 

4.6 

8.2 

c+ 

11.5 

16.6 

2.2 

4.2 

7.3 

10.9 

c 

26.9 

29.6 

10.6 

14.1 

23.7 

27.4 

c— 

30.3 

25.2 

27.6 

25.6 

34.8 

29.6 

D 

17.4 

11.7 

40.1 

36.9 

19.3 

14.5 

D— 

8.1 

5.8 

17.4 

15.7 

8.5 

5.9 

White  Troops  (Heavy  Territory) 

A 

1.1 

2.6 

0.6 

0.9 

1.3 

2    » 

B 

4.8 

7.4 

0.9 

2.1 

4.6 

7.3 

c+ 

12.3 

17.1 

2.2 

3.8 

7.0 

10.3 

c 

27.0 

30.7 

10.0 

14.0 

22.5 

25.7 

c- 

30.7 

24.8 

27.9 

26.0 

34.8 

29.7 

D 

16.8 

11.3 

41.9 

38.2 

21.0 

15.  i 

D— 

7.3 

6.1 

16.5 

15.0 

8.8 

6.6 

White  Troops  (Light  Territory) 

A 

1.4 

3.6 

1.2 

2.3 

1.8 

4.7 

B 

4.0 

8.7 

1.9 

2.5 

4.6 

9.8 

c+ 

10.4 

16.0 

2.3 

6.0 

8.1 

12.1 

c 

26.8 

28.1 

12.9 

15.0 

26.2 

28.7 

c— 

29.8 

25.8 

26.7 

24.0 

35.0 

29.5 

D 

18.2 

12.3 

34.2 

31.0 

16.2 

10.8 

D— 

9.4 

5.5 

20.8 

19.2 

8.0 

4.4 

Colored  Troops 

A 

0.0 

0.0 

0.7 

1.3 

0.7 

1.6 

B 

1.8 

1.5 

0.7 

2.9 

1.1 

2.0 

c+ 

1.8 

3.5 

1.4 

3.6 

1.0 

2.8 

c 

6.2 

10.4 

5.2 

5.2 

5.0 

7.2 

c— 

17.3 

14.8 

11.0 

11.4 

15.2 

18.8 

D 

16.7 

22.3 

37.2 

33.3 

38.4 

35.9 

D— 

56.2 

47.5 

43.8 

42.2 

38.6 

31.7 

216  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 


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218  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 

Table  I  shows  the  comparative  distribution  by  letter  ratings 
of  hookworm  and  non-hookworm  groups  for  Alpha,  Beta  and 
"  Common  Scale,"  for  all  white  troops,  for  white  troops  of 
light  territory  and  of  those  of  heavy  territory,  and  for  all 
colored  troops.  Median  numerical  scores  for  Alpha  and  for 
Beta,  median  mental  age  (Common  Scale),  median  chrono- 
logical age  and  median  number  of  years  attending  school  are 
given  under  corresponding  legends  in  Table  II.  These  dis- 
tributions of  ratings  of  ages  and  years  of  schooling  with  their 
respective  medians  are  all  presented  graphically  in  plates  1-22. 

Furthermore,  all  the  hookworm  troops  are  compared  with 
all  the  non-hookworm  troops  in  respect  to  successive  10% 
increments  in  Alpha,  Beta  and  "  Common  Scale,"  as  shown 
in  plates  17-22. 

Data  from  which  these  graphs  are  constructed  are  given  in 
Table  III. 

ANALYSIS  AND  INTERPRETATION 

By  reference  to  Tables  I  and  II  it  is  found  that  the  intelli- 
gence of  hookworm  troops  is  lower  than  for  the  non-hook- 
worm troops;  that  they  have  fewer  years  of  schooling;  and 
that  they  are  younger  chronologically.  Mere  inspection  of 
the  tables  show  these  differences  to  be  unmistakable  and  to  be 
based  on  such  a  large  number  of  cases  as  not  to  warrant  com- 
putation of  a  mathematical  probable  error  of  these  differences.2 
Furthermore,  all  these  differences  are  greater  for  white  troops 
from  "  light "  territory  than  for  those  from  "  heavy  "  terri- 
tory and,  in  most  cases,  greater  for  the  white  troops  than  for 
the  colored  troops. 

Moreover,  when,  according  to  Table  III  and  graphs,  plates 
17-22,  hookworm  and  non-hookworm  troops  are  compared  by 
average  score  of  successive  10%  steps  from  the  highest  to  the 
lowest,  it  is  clear  that  the  difference  in  intelligence  rating 
between  hookworm  and  non-hookworm  troops  of  the  lowest 
level  of  intelligence  is  comparatively  slight.  From  this  it  is 
safe  to  infer  that  the  lower  the  intellectual  scale,  the  less  the 
relative  inferiority  of  the  hookworm  troops. 

So  much  for  the  data.  What  do  they  mean  ?  Before  offer- 
ing an  interpretation,  let  there  be  a  critical  survey  of  earlier 

2  It  should  be  noted  that  the  sum  of  the  Alpha  cases  and  of  the 
Beta  cases  of  hookworm  and  non-hookworm  groups,  respectively,  do 
not  quite  equal  the  total  number  for  each  group  in  the  "  Common 
Scale"  column,  since  the  difference  was  made  up  by  the  individually 
examined  cases.  Likewise  the  total  numbers  for  the  age  and  schooling 
columns  do  not,  as  they  should,  equal  the  totals  of  the  Common  Scale 
column,  since  some  troops  failed  to  note  their  age,  some  their  schooling, 
and  some,  both. 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  219 

studies  of  similar  problems.     The  first  study  designed  to  meas- 
ure the  influence  of   hookworm  disease  on  intelligence   was 
made  on  school  children  by  E.  K.  Strong  under  the  auspices 
of  the  Rocke!iHler  Foundation,  in  1916.3 
From  his  study,  Strong  concludes: 

1.  "  Hookworm  disease  interferes  very  radically  with  men- 
tal development." 

2.  "  Treatment  alleviates  this  condition  to  some  extent,  but 
it  does  not,  immediately  at  least,  permit  the  child  to  gain  as  he 
would  if  he  had  not  had  the  disease." 

3.  "  The  longer  the  child  has  the  hookworm  disease,  the 
less  will    be  the  improvement    mentally   when  the   child    is 
treated." 

4.  The  last  statement  Strong  considers  as  "  probably  the 
most  important  deduction  from  this  whole  study." 

Such  are  Strong's  conclusions,  which  corroborate  the  aver- 
age man's  casual  observation  of  the  apparent  effect  of  hook- 
worm disease  and,  in  support  of  which,  all  who  are  interested 
in  stamping  out  the  disease  are  eagerly  waiting  for  conclusive 
scientific  data.  But  neither  Strong  nor  anyone  else  has  yet 
produced  such  data. 

Strong  compared  the  gains  made  in  several  of  the  then  most 
highly  standardized  single  group  intelligence  tests  after  a 
period  of  three  and  a  half  months  by  the  following  group  of 
school  children: 

18  children  without  hookworm. 
9  untreated  hookworm  children. 
27  completely  cured  hookworm  children. 
17  incompletely  cured  hookworm  children. 

All  these  children  were  given  the  Binet-Simon  Test,  in 
addition  to  seven  other  mental  tests,  calculation  test,  logical 
memory,  opposite  test,  memory  span,  handwriting,  form-board 
test,  all  of  which,  together  with  the  Binet  test  were  repeated 
after  an  interval  of  three  and  a  half  months.  The  difference 
between  chronological  age  and  Binet  age  for  the  respective 
groups  was  found  to  be:  non-hookworm  children,  — 1.0  year; 
completely  cured  hookworm  children,  — 1.6  years;  incom- 
pletely cured  hookworm  children,  — 2.2  years;  and  un- 
treated hookworm  children,  — 1.2  years.  According  to 
the  chronological  age  distribution  for  the  various  groups,  all 
the  non-treated  hookworm  children  fall  between  the  ages  9.9- 
10.9  years,  while  all  the  other  groups  are  rather  evenly  divided 
between  the  age  intervals  9.9-10.9  years  and  11-12  years. 

3  Effect  of  Hookworm  Disease  on  the  Mental  and  Physical  Develop- 
ment of  Children.  N.  Y.  The  Rockefeller  Foundation,  1916. 


220  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 

Now  it  has  been  pointed  out  that  for  ages  above  10.5  or  10 
years,  mental  age  cannot  be  accurately  determined  by  the  old 
Binet  test,  which  fact  Strong  himself  records  in  a  footnote 
of  his  study  (page  74).  Of  this  shortcoming  of  the  old  un- 
revised  Binet  test  which  was  used  by  Strong,  Terman  has  to 
say  the  following:  "The  proportion  of  feeble-mindedness 
among  adult  subjects  was  greatly  overestimated,  because  sub- 
jects who  were  really  of  the  12  or  13  years  mental  level  could 
only  earn  a  mental  age  of  about  11  years."*  He  finds  fur- 
thermore that  young  subjects  get  too  high  a  mental  age :  "  In 
young  subjects  the  higher  grades  of  mental  deficiency  were 
overlooked,  because  the  scale  caused  such  subjects  to  test  only 
a  little  below  normal." 

There  is  a  very  high  probability  then  that  Strong's  mental 
ages  are  not  the  true  mental  ages  and  that  the  non-treated 
hookworm  group,  a  comparatively  young  group,  had  a  mental 
age  deficiency  of  considerably  over  1.2  years;  and  that  since 
all  the  other  groups  fall  pretty  heavily  into  the  11  to  12  age 
group,  their  mental  age  deficiency  found  by  Strong  is  too 
great.  Indeed,  on  the  basis  of  the  age  distribution  of  the 
groups  it  is  highly  probable  that  for  increasing  order  of  mental 
age  deficiency  the  groups  stand :  non-hookworm  children,  com- 
pletely cured  hookworm  children,  incompletely  cured  hook- 
worm children  and  untreated  hookworm  children. 

Strong's  data  on  the  social  status  of  the  groups  help 
corroborate  this  probability,  for  of  the  non-hookworm  chil- 
dren 94%  came  from  the  best  grade  of  homes,  0%  from 
the  poorest.  Of  the  completely  cured  hookworm  children 
41%  came  from  the  best  grade  of  homes,  15%  from  the 
poorest.  Of  the  incompletely  cured  hookworm  children  12% 
came  from  the  best  grade  of  homes,  29%  from  the  poorest. 
Of  the  untreated  hookworm  children  33%  came  from  the  best 
grade  of  homes,  56%  from  the  poorest.  This  is  considered 
a  corroboration  of  the  assumption  thatjbetter  social  ^rrdrpn- 
ment  on  the  whole  presupposes  a  higher  level  of  intelligence 
in  that  environment.  Considerable  data5  are  available  in  sup- 
port of  the  latter  assumption. 

It  so  happened  that  on  the  whole  the  gains  in  the  second 
trial  of  the  several  mental  tests  (other  than  Binet)  classified 
the  groups  in  the  order,  non-hookworm  children,  completely 
cured  hookworm  children,  incompletely  cured  hookworm  chil- 
dren and  untreated  hookworm  children,  which  is  the  same 

*  The  Measurement  of  Intelligence,  Lewis  M.  Terman,  p.  3. 
5  Lewis  M.  Terman.  The  Measurement  of  Intelligence,  pp.  72-73. 
Robert  M.  Yerkes,  James  W.  Bridges,  Robert  S.  Hardwick,  A  Point 
Scale  for  Measuring  Mental  Ability. 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  221 

order  as  the  very  highly  probable  mental  age  order  of  the 
groups  to  start  with,  as  was  shown  above.  //  the  latter  be 
true,  then  the  order  of  gain  is  but  to  have  been  expected,  hook- 
worm or  no  hookworm.  It  is  now  generally  accepted  as  a 
fact  that  the  more  intelligent  learner  learns  the  more  rapidly. 
Strong*  himself  has  furnished  data  elsewhere  which  he  ap- 
parently had  forgot  when  interpreting  his  data  on  hookworm. 
He  concludes  from  an  experiment  on  school  children: 

"  The  slope  of  learning  curves  of  school  children  based  on 
simple  arithmetical  combinations  apparently  correlates  to  a 
very  considerable  extent  with  the  general  intelligence  of  the 
children."  A  few  years,  previously  Colvin,  practicing-  five 
normal  subjects  with  five  subnormal  at  canceling  A's,  found 
that  "  in  every  case  the  normal  child  made  greater  improve- 
ment with  less  fluctuation  than  did  the  subnormal  child."7 

Likewise  the  writer  found  from  a  card  sorting  practice  on 
27  normal  school  students  for  50  minutes  a  day,  with  the 
practice  repeated  after  10  days,  11  days  and  Sl/2  months,  re- 
spectively, a  positive  correlation  of  -(-.48  between  maximum 
gain  and  intelligence,  when  there  was  no  correlation  between 
initial  performances  and  intelligence.  This  indicated  pretty 
clearly  that  the  +.48  correlation  showed  a  very  real  positive 
relationship  between  intelligence  and  learning  ability.  It 
should  be  noted  that  "  intelligence  "  was  determined  by  having 
each  student  rank  all  the  other  students  of  the  class  on  the 
basis  of  her  estimate  of  each  student's  intelligence,  from  which 
a  combined  ranking  was  computed.8 

Assuming,  then,  that  the  four  groups  studied  by  Strong  had 
at  the  beginning  of  the  study  mental  capacity  in  the  order, — 
non-hookworm  children,  completely  cured  hookworm  children ; 
incompletely  cured  hookworm  children,  and  untreated  hook- 
worm children;  which  his  difference  between  chronological 
and  mental  ages  (p.  21)  indicates  when  these  data  are  inter- 
preted in  the  light  of  Terman's  Revision  of  the  Binet  tests, 
that  the  gain,  therefore,  for  these  groups  after  $l/>  months 

6  Learning  Curve  as  a  Diagnostic  Measure  of  Intelligence, — Psycho- 
logical Bulletin,  1917,  Vol.  14  pp.  153-154. 

7  S.    S.    Colvin,    Notes    on    Certain    Aspects    of '  Learning — Psycho- 
logical Bulletin,  Feb.  15,  1915,  Vol  12. 

8  Garry  C.  Myers,  Some  Variabilities  and  Correlation  in  Learning. 
Amer.  Jr.  Psychol.  July,  1918,  Vol.  29  pp.  316-326. 

Since  completing  this  study  the  writer  has  demonstrated  in  a  prac- 
tical way  that  learning  progress  is  in  proportion  to  intelligence  ratings, 
in  a  school  of  1,800  illiterates  whom  he  classified  on  the  basis  of  intelli- 
gence ratings.  See  Principles,  Plans  and  Purposes  of  the  Recruit 
Educational  Center,  Camp  Upton,  N.  Y.,  War  Department,  Washing- 
ton, D.  C, 


222  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 

should  show  exactly  that  order  is  just  what  should  be  expected, 
regardless  of  hookworm.  Apparently,  then  all  Strong's  study 
has  shown  is  that  brighter  children  learn  faster  than  duller 
children. 

It  must  be  said  of  Strong's  work  that  his  method  of  at- 
tempting to  measure  the  effect  of  hookworm  on  mental  de- 
velopment in  terms  of  relative  learning  xgains  by  hookworm 
and  non-hookworm  children  is  highly  commendable.  His 
method,  if  refined  by  comparing  groups  of  equal  mental  and 
social  status,  on  the  basis  of  their  learning  rate  as  measured 
by  learning  curves  developed  with  considerable  practice  on 
specific  learning  tasks  and  on  large  number  of  children, 
should  help  measure  with  a  good  deal  of  exactitude  the  influ- 
ence of  hookworm  disease  on  intelligence. 

Strong  also  studied  11  children — not  included  in  the  part 
reviewed  above ;  who  on  the  average  were  13.5  years  of  age, 
comparing  this  group  with  the  17  incompletely  cured  hook- 
worm children  averaging  11.1  years  chronologically,  both 
groups  coming  "  from  homes  of  approximately  the  same  eco- 
nomic conditions."  The  average  mental  ages  for  the  respec- 
tive groups,  by  Binet-Simon  tests,  were  found  to  be  9.0  years, 
and  8.5  respectively,  from  which  are  derived  intelligence  quo- 
tients of  .67  and  .77.  Of  course,  here  again  the  real  differ- 
ence is  not  quite  so  great  if  the  error  of  Binet  for  the  older 
children  is  taken  into  account.  Nevertheless,  it  is  obvious 
that  the  older  group  of  11  children,  as  pointed  out  by  Strong, 
is  appreciably  lower  intellectually,  which  fact  would  presup- 
pose a  lower  rate  of  gain  by  them,  which  gain  was  1.5%  as 
against  5.9%  for  the  younger  group.  "  When  the  two  groups 
are  compared  with  normal  children,  we  see  that  the  younger 
children  have  gained  but  34%  of  what  the  healthy  children 
accomplished,  while  the  older  children  have  gained  but  9%." 
It  should  be  recalled  that  this  "  normal "  group  had  an  aver- 
age chronological  age  of  11.1  years  and  mental  age  of  10.1 
years,  giving  an  intelligence  quotient  of  .91.  Certainly,  there- 
fore, Strong  is  not  justified  because  the  older  hookworm  chil- 
dren with  lower  intelligence  gain  less  than  the  younger  hook- 
worm children  of  higher  intelligence,  in  concluding :  "  The 
longer  the  child  has  the  hookworm  disease  the  less  will  be  the 
improvement  mentally  when  the  child  is  treated"  And  not 
justified  in  spite  of  the  fact  that  he  notes,  "  Our  careful  study 
made  of  these  children  (the  13.5  year  old  children)  did  not 
reveal  any  other  cause  for  the  retardation  than  hookworm 
disease."  Here  again  he  has  merely  shown  by  his  data  that 
the  brighter  children  learn  faster  than  the  duller  children, 
which  is  to  be  expected.  Furthermore,  it  is  not  clear  on  what 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  223 

ground  Strong  assumes  that  the  13.5  year  old  children  with 
hookworm  have  had  the  disease  longer  than  the  11.1  year  old 
children. 

In  1917  Truman  Lee  Kelly9  reported  a  study  in  which  his 
results  disagree  with  those  of  Strong's  to  the  effect  that  chil- 
dren cured  of  hookworm  improve  more  rapidly  in  mental 
traits  after  cure  than  do  the  normal  children. 

He  too  had  a  small  number  of  cases  in  the  groups  compared : 

47  children  free  from  malaria  and  hookworm. 

9  children  cured  of  hookworm. 

11  children  cured  of  malaria. 

11  children  cured  of  malaria  and  hookworm. 

The  tests  were  Starch,  Arithmetic,  Courtis  Arithmetic, 
Trabue  completion  and  Thorndike  Reading  tests,  which  were 
repeated  after  a  six  months'  interval.  Kelly  gives  no  indica- 
tion as  to  the  mental  status  of  the  group  compared  at  the  time 
of  the  first  test.  Furthermore,  since  he  depended  upon  the 
children's  report  as  to  whether  or  not  they  had  malaria  one 
is  not  sure  this  factor  is  wholly  eliminated  from  the  "  f ree- 
f  rom-malaria  and  hookworm  "  group. 

If  number  of  cases  were  the  solution  of  this  problem,  the 
solution  should  have  been  near  at  hand  upon  the  report  of 
Waite's  Study,10  wherein  he  tested  116  non-infected  children, 
65  lightly  infected  children  and  159  heavily  infected  children, 
with  Goddard's  Revision  of  the  Binet-Simon  Tests,  and  the 
Porteus  Mazes.  Waite  classified  his  group  in  this  way : 

"  1.  The  heavily  infected  cases  presenting  hookworm  ova 
in  plain  smear  examinations  of  stools ; 

"  2.  The  lightly  infected  cases,  presenting  hookworm  ova 
only  in  smears  of  centrifuged  stools ;  and 

"  3.  The  non-infected  cases,  who  showed  no  ova  in  four 
successive  stool  examinations,  two  of  plain  smears,  and  two 
of  centrifuged  smears." 

Expressed  in  mental  age  the  retardations  for  the  respective 
groups  for  Binet  and  Porteus  tests  were  as  follows: 

9  Effect    of    Malaria    and    Hookworm    upon    Physical    and    Mental 
Development  of  School  Children.     Elementary  School  Journal,  Vol.  18, 
1917,  pp.  43-55. 

10  Dr.  G.  H.  Waite,  A  Study  of  the  Effects  of  Hookworm  Infection 
upon  the  Mental  Development  of  North  Queensland  School  Children. 
The  Australian  Medical  Journal  Jan.  1919.    The  original  manuscript 
dated  1918  was  loaned  the  writer  through  Major  John  L.  Riley,  by 
the  Library  of  the  Rockefeller  Foundation. 


224  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 


Retarded  by  Binet... 

Non-Infected 
3.  9  mo. 

Lightly  Infected 
9.  3  mo. 

Heavily  Infected 
23.  4  mo. 

Retarded  by  Porteus 

2.  7  mo. 

4  .  7  mo. 

16.  00  mo. 

Unfortunately  Waite  computed  the  average  retardation 
rather  than  the  median,  thereby  giving  undue  weight  to  the 
extremely  low  cases.  Nevertheless,  these  data  show  conclu- 
sively that  the  hookworm  children  studied  are  on  the  average 
more  retarded  mentally  than  the  non-hookworm  children,  and 
the  heavily  infected  group  is  more  retarded  than  the  lightly 
infected  group.  But  they  do  not  conclusively  support  the  in- 
ferences derived  therefrom  by  Waite,  wherein  he  concludes: 

1.  "  The  results  bring  to  light  clearly  two  features,  namely, 
that  hookworm  infection  produces  in  growing  children  severely 
arrested  mental  development,  and  considerable  mental  slug- 
gishness." 

2.  "  The  duration  of  hookworm  infection  is  important,  be- 
cause the  longer  the  infection  persist  the  greater  is  the  mental 
retardation." 

3.  "  The  degree  of  hookworm  infection,  and  therefore  the 
amount  of   anemia,   definitely  influences  the  amount  of   re- 
tardation of  mental  development." 

4.  "  The  most  backward  hookworm  children  invariably  give 
the  history  of  an  early  infection." 

Considering  Waite's  first  point,  there  arises  the  ever-present 
problem  in  interpretation  of  such  data  which  he  seems  wholly 
to  ignore,  namely,  how  is  anyone  to  know  from  any  data  which 
merely  shows  the  hookworm  group  inferior  in  mental  rating 
whether  the  inferiority  is  due  to  hookworm  or  whether  because 
of  their  inferiority  such  children  were  of  poor  sanitary  habits, 
thereby  more  exposed  to  the  infection?  In  other  words,  to 
what  degree  does  hookworm  select  the  lower  level  of  intelli- 
gence? Certainly  Waite  is  not  justified  by  the  data  he  fur- 
nishes in  making  his  first  conclusion.  On  the  basis  of  his 
data  one  could  infer  with  just  as  much  logic  that  "  severely 
arrested  mental  development,  and  considerable  mental  slug- 
gishness "  tended  to  make  the  children  easy  victims  to  hook- 
worm. 

The  validity  of  Waite's  succeeding  inferences  hangs  upon 
that  of  his  first  one. 

In  the  study  of  Major  Kofoid  and  Pittenger  (see  this  paper, 
p.  211),  their  comparative  data  expressed  in  10%  steps  from 
the  highest  to  the  lowest  for  hookworm  and  for  non-hookworm 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  225 


cases,  respectively,  indicate  an  even  more  pronounced  tendency 
at  convergence  toward  the  lower  extreme  than  does  this  study. 
Here  are  their  data  for  Alpha: 


501  Hookworm 

Cases 
Aver.  Score 


1st  (Highest)  10% 
2d 
3d 
4th 
5th 
6th 
7th 
8th 
9th 
10th  (Lowest) 

Median  score. 


181.0 

146.6 

118.6 

99.0 


80, 
63, 
48, 
36. 
24.6 
1.0 


4,792  Non-Hookworm 

Cases 

Aver.  Score 
219.0 
178.5 
150.6 
127.7 
107.4 


89.6 
70.7 
51.3 
31.8 
0.0 


Although  their  data  were  given  in  weighted  scores  which 
are  about  twice  the  unweighted  scores  in  the  present  study, 
thereby  rendering  the  two  sets  of  data  not  wholly  comparable, 
yet  the  general  tendency  is  the  same  as  that  shown  by  our 
data.  Of  course  their  relative  inferiority  of  the  hookworm 
group  is  very  much  greater  than  that  for  our  study,  they  also 
found  a  greater  increase  of  difference  from  the  lowest  mental 
level  to  the  highest.  This  is  to  be  expected  since  their  nega- 
tive group  was  selected  at  random  including  troops  undoubt- 
edly from  the  larger  urban  populations  and  other  non-infected 
or  lightly  infected  areas,  whereas  each  of  our  hookworm  cases 
was  paired  with  a  non-hookworm  case  from  the  same  local 
territory.  They  also  found  in  accord  with  our  data,  that  the 
relative  inferiority  of  the  hookworm  troops  was  much  greater 
for  whites  than  for  colored  and  greater  also  for  Alpha  troops 
than  for  Beta  troops. 

Interpreting  these  data  Kofoid  and  Tucker  say :  "  In  liter- 
ate blacks  the  difference  is  less,  as  might  be  expected,  because 
of  a  relative  racial  immunity."  They  conclude: 

"The  interpretation  of  the  data  is  that  men  infected  with 
hookworm  belong  to  all  classes  of  mental  rating,  but  not 
wholly  proportionately  in  all  groups  to  those  without  the  in- 
fection, the  men  with  the  low  scores  showing  a  greater  relative 
deficiency  as  the  score  falls.  It  also  appears  that  the  infected 
men  are  pushed  en  masse  below  the  normal  by  infection  by 
hookworm,  or  by  other  factors  correlated  therewith  in  a  vicious 


226  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 

circle,  but  that  the  men  with  lower  scores  have  a  greater 
momentum  and  move  relatively  farther  from  the  norm  than 
do  infected  men  with  higher  scores.  This  suggests  the  con- 
clusion that  men  with  hookworm  include  a  greater  proportion 
of  subnormal  types  than  men  without  and  that  subnormal  men 
tend  to  acquire  the  disease  more  rapidly  than  men  of  higher 
categories.  It  is  also  possible  that  the  intensity  of  individual 
infections  may  be  heavier  in  the  groups  of  lower  intelligence, 
due  to  the  relation  of  intelligence  to  sanitation,  which  in  turn 
modifies  the  chances  of  infection  and  thus  the  ultimate  in- 
tensity thereof." 

As  for  the  negroes,  they  logically  fit  into  the  lower  order, 
consequently  showing,  as  do  the  lower  whites,  a  comparatively 
small  inferiority  for  the  hookworm  group.  Then  the  question 
may  arise  why  is  not  the  convergence  consistent  from  the 
highest  to  the  lowest  extremes  of  intelligence  instead  of  a 
rather  sudden  convergence  toward  the  lower  extreme?  The 
answer  may  be  that  with  the  exception  of  the  lowest  30  or 
40%  of  the  intelligence  scale  of  a  community,  there  is  hardly 
to  be  expected  a  great  improvement  in  sanitary  habits  near 
the  more  intellectual  extreme.  Certainly  the  degree  of  rise 
in  standards  is  hardly  proportional  all  the  way  upward.  On 
the  other  hand,  just  why  should  the  very  upper  extreme  of 
the  hookworm  and  non-hookworm  groups  be  actually  closer 
than  the  mean  where  there  is  a  slight  bulging  in  the  space 
between  the  curves?  Undoubtedly  this  is  due  to  a  failure  of 
Alpha  to  differentiate  as  widely  in  intelligence  scores  for  the 
upper  extremes  of  intelligence.  In  the  numerous  studies  of 
Alpha  by  the  Division  of  Psychology  it  was  invariably  found 
that,  on  the  basis  of  a  man's  military  value,  Alpha  did  not 
clearly  discriminate  between  the  upper  and  lower  A  grade 
man,  nor  even  always  between  the  A  and  B  man.11  Further- 
more when  given  to  college  men,  the  discrimination  in  terms 
of  intelligence  rating  is  certainly  not  very  pronounced.12 

On  the  other  hand,  the  convergence  at  the  lower  extreme 
may  mean  that  hookworm  disease  in  some  peculiar  way  does 
not  affect  persons  of  very  low  mentality  or  even  those  of  very 
high  mentality  or  that  the  effect  is  so  small  as  to  be  registered 
but  slightly  or  not  at  all  by  an  intelligence  test.  In  this  event 
the  difference  between  the  hookworm  and  non-hookworm 
groups  would  undoubtedly  be  due  to  hookworm  infection. 

If  two  groups,  say  from  northern  states  where  hookworm 

11  Psychological   Examining  in   the  United   States  Army,   Part   III. 
P.  published  by  the  National  Academy  of  Sciences,  Vol.  XV. 

12  School  and  Society,  Vol.  X,  No.  250,  pp.  437-440. 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  227 

is  practically  absent,  whose  median  mental  status  were  known 
to  be  quite  disparate,  were  compared  on  the  basis  of  10% 
steps,  comparison  with  such  data  would  help  clear  up  the 
difficulty  in  interpreting  the  lack  of  complete  parallelism  be- 
tween the  10%  steps  of  the  hookworm  and  non-hookworm 
groups  of  this  study. 

In  reading  the  data  of  Table  I  care  should  be  exercised  in 
interpreting  per  cents  of  difference  in  respect  to  all  mental 
ratings  since  a  year's  difference  between  mental  ages  7  and  8 
years  cannot  mean  the  same  as  a  year's  difference  between  11 
and  12  years,  nor  in  terms  of  per  cent,  since  even  in  spite  of 
intelligence  scales  there  is  no  certainty  that  the  increments  in 
terms  of  mental  age  or  even  in  terms  of  Alpha  or  Beta  scores 
are  constant.  Let  such  per  cents  merely  be  read  as  ratios  to 
the  higher  values  compared  and  no  more. 

RELATION  OF  SCHOOLING  TO  HOOKWORM 
When  the  amount  of  schooling  reported  is  considered  the 
data  are  no  more  conclusive  in  what  they  mean,  for  very  high 
correlations  were   found  between  the  army  group  tests  and 
amount  of  schooling.13 

Just  what  the  high  correlation  between  intelligence  rating 
and  schooling  means  is  not  certain,  i.  e.,  one  does  not  know 
how  much  schooling  offsets  ability  to  score  in  the  army  tests 
nor  to  what  degree  the  amount  of  native  ability  determined  how 
long  school  had  been  attended.  In  this  study  exactly  the  same 
problem  arises:  Did  hookworm  disease  decrease  the  amount 
of  school  attendance  or  was  the  kind  of  children  whose  school 
attendance  was  limited,  of  those  children  who  most  readily 
fell  heir  to  the  disease  ?  Maybe  such  children  were  those  less 
likely  to  have  shoes  and  not  having  so  much  schooling  and 
not  coming  perhaps  from  so  good  a  social  environment  would 
not  have  developed  so  good  sanitary  habits. 

RELATION  OF  AGE  TO  HOOKWORM 

Since  all  the  troops  from  among  whom  the  cases  selected 
were  of  practically  the  same  age  distribution,  it  is  quite  sig- 
nificant that  the  hookworm  troops  are  from  .7  to  .9  of  a  year 
younger  than  the  non-hookworm  troops. 

According  to  the  report  of  the  Porto  Rico  Commission,14 
there  was  found  the  following  age  distinction : 

13  Psychological  Examining  in  the  Army,  Part  III.     National  Acad- 
emy of  Sciences,  Vol.  XV. 

14  Ashford  and  Jgaravidez,  Uncinariasis  in   Porto  Rico,   Sen.   Dec. 
808,  Washington,  D.  C,  1911. 


228  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 

Age  Cases 

Under  10  years   15,622 

10  to  20  years 50,924 

21  to  30  years 36,589 

31  to  40  years 18,254 

41  to  50  years S,/96 

51  to  60  years   3,841 

Over  60  years 1,413 

Although  the  cases  examined  were  not  exactly  a  random 
sampling,  these  figures  doubtless  represent  the  approximate 
age  distribution,  and  are  slightly  suggestive  of  a  corroboration 
of  our  findings. 

A  study  reported  by  Major  Siler  and  Major  Cole  in  1917 
is  more  to  the  point.15 

Of  the  First  Mississippi  Infantry  32%  were  infected,  and 
the  First  Alabama  Cavalry,  54%.  The  age  distribution  for 
the  highest  10  years  were  as  follows: 

First  Mississippi    First  Alabama 

Years  %  % 

16-17 1  2 

18-19   21  42 

20-21    32  24 

22-23 20  12 

23-24   11  7 

Obviously  the  First  Alabama  Infantry,  a  greater  per  cent 
of  whose  troops  have  hookworm,  are  younger  than  those  of 
the  First  Mississippi.  In  the  light  of  these  data  the  writers 
inferred  (p.  95)  : 

"  In  our  opinion  there  is  but  little  or  no  doubt  that  age 
accounts  to  some  extent  for  the  greater  prevalence  of  both 
hookworm  disease  and  measles  in  the  First  Alabama  Cavalry." 

From  this  data  and  from  the  present  study  (see  plate  25-28) 
it  seems  that  the  younger  troops  are  more  susceptible  to  hook- 
worm disease  than  the  older  troops.  Certainly  more  data  is 
desirable  as  to  the  exact  distribution  of  hookworm  cases  on 
the  basis  of  total  population  at  various  ages  and  in  various 
racial  levels.  However,  it  is  also  probable,  since  hookworm 
infection  is  limited  in  its  length  of  life  within  a  given  indi- 
vidual, that  many  of  the  older  men,  who,  in  the  hookworm 

15  Major  J.  F.  Siler,  M.  C.  and  Major  C.  L.  Cole,  M.  C,  The 
Prevalence  of  Hookworm  Disease  in  the  Fourth  Texas  Infantry.  Fir^t 
Mississippi  Infantry,  and  First  Alabama  Cavalry  Regiment.  The 
Military  Surgeon,  1917,  Vol.  41,  pp.  77-99. 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  229 

survey  fell  into  the  non-hookworm  group,  had  once  been  hook- 
worm cases  but  had  become  free  from  hookworm  at  some 
time  previous  to  the  hookworm  examination. 

CONCLUSION 

The  average  person  of  a  given  community,  who  is  infected 
with  hookworm,  is  inferior  mentally  to  the  average  person, 
of  the  same  community,  who  is  not  infected  with  hookworm. 
In  support  of  this  statement  conclusive  data  have  been  pre- 
sented. 

But  there  are  yet  no  data  available  to  show  conclusively 
whether  this  mental  inferiority  is  due  to  hookworm  disease 
or  whether  hookworm  disease  is  due  to  mental  inferiority. 

A  fact  not  calling  for  evidence  here  is  that  hookworm  dis- 
ease is  a  social  and  economic  burden  upon  tropical  and  sub- 
tropical countries.  Now  if  it  does  not  in  and  of  itself  lower 
the  mentality  of  its  victim,  there  is  ample  evidence  that  it  will, 
as  will  any  other  disease,  diminish  his  educational  opportuni- 
ties, and  thereby  his  efficiency^as  a  social  unit. 

Since  because  of  the  very  nature  of  the  transmission  of 
hookworm  disease,  wholly  dependent  upon  remedial  sanitary 
habits  and  conditions,  doubtless  it  is  true  that  just  because  of 
lower  intelligence  or  lower  mental  training  the  victim  more 
readily  falls  heir  to  the  disease;  hence  the  obvious  remedy  is 
more  education  to  the  educable,  a  far  stricter  governmental 
guardianship  over  the  non-educable  and  over  those  educable 
to  but  a  slight  degree,  and  an  aggressive  general  governmental 
control  over  the  social  habits  and  sanitary  conditions  of  the 
community.  In  matters  of  education,  mere  ability  to  master 
the  three  R's  should  not  be  the  goal  alone,  but  through  this 
median  specific  instruction  in  matters  of  simple  rules  of  health 
and  sanitation  is  of  imperative  necessity. 

This  emphasis  upon  specific  hygiene  instructions  is  obviously 
none  the  less  imperative  if  it  is  discovered  that  the  hookworm 
infection  is  primarily  the  cause  of  the  relatively  lower  mental 
status  of  its  victims. 

So,  then,  whether  hookworm  disease  is  the  cause  of  mental 
inferiority  or  the  result,  or  both,  the  fact  that  those  infected 
with  the  disease  are,  as  a  rule,  distinctly  inferior  to  those  free 
from  the  disease,  as  shown  conclusively  by  this  study,  is  an 
inexorable  challenge  to  the  federal,  state  and  local  leaders  in 
matters  of"  health  education  and  general  economic  and  social 
betterment,  to  use  every  possible  means  to  rid  the  country  of 
this  pesf 


230  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 

SUGGESTIONS  FOR  FURTHER  RESEARCH 

The  question  remaining  to  be  answered  is,  Does  hookworm 
disease  directly  depreciate  the  mental  status  of  its  victims? 

It  has  been  seen  that  Strong's  method  of  comparing  gains 
in  learning  by  the  cured-of-hookworm  children  with  hookworm 
children,  while  on  the  right  track  apparently,  was  fallacious 
in  that  it  ignored  the  fact  that  ability  to  learn  correlates  highly 
with  general  intelligence;  and,  that  as  subsequent  studies  and 
as  his  own  data  indicate,  almost  any  randomly  selected  hook- 
worm group  will  be  found  to  be  of  lower  mental  status  than 
a  non-hookworm  group  similarly  selected. 

It  is  necessary,  therefore,  in  order  to  make  a  fair  compari- 
son, to  be  sure  that  the  groups  compared  are  at  the  time  of  the 
first  learning  test  of  practically  the  same  mental  ability  with 
practically  equal  distribution  of  abilities  in  the  groups  com- 
pared. 

Furthermore,  the  social  status  of  the  compared  groups 
should  be  equal.  A  study  of  infected  and  non-infected  chil- 
dren of  the  same  family  (especially  of  twins)  would  meet 
both  conditions  fairly  well.  That  the  social  status  be  equal- 
ized in  such  cases  would  be  obvious  and  that  twins  show  far 
closer  mental  resemblance  than  siblings  and  siblings  far  closer 
resemblance  than  randomly  selected  children,  has  been  well 
established  by  Galton,1*5  Thorndike,17  Rusk,18  Cobb,19  Starch,20 
and  others. 

In  the  absence  of  either  class  of  subjects  to  be  studied,  the 
group  intelligence  tests  available  would  enable  the  selection 
of  groups  of  practically  the  same  mental  abilities. 

If,  then,  of  those  two  groups,  the  hookworm  group  learned 
appreciably  more  slowly  than  the  non-hookworm  group  or 
cured-of-hookworm  group,  the  obvious  inference  would  be 
pretty  safe. 

The  learning  task  should  extend  over  a  period  of  several 
weeks  or  months  including  numerous  practices,  thereby  offer- 
ing opportunities  to  measure  mental  endurance  and  attitude 
toward  a  task  whose  novelty  has  worn  off  and  wherein  the 
stimulation  of  a  stranger  as  examiner  has  ceased  to  function 
as  a  stimulation. 

16  Sir  Francis  Galton,  History  of  Twins,  p.  170  f  of  the  reprint  in 
Everyman's  Library. 

17  E.  L.  Thorndike,  Measurement  of  Twins,  Archives  of  Philosophy, 
Psychology  and  Scientific  Methods  No.  1,  1905. 

18  Elizabeth  Rusk,   Mental  Resemblance   among  Siblings.     Teachers 
College,  1908,  Masters  Thesis,  (unpublished) 

19  Margaret  V.   Cobb,  A   Preliminary   Study  of  the   Inheritance  of 
Arithmetical     Abilities.    Journal     of     Educational     Psychology     1917, 
Vol.  8 —pp.  1-20. 

20  Daniel  Starch,  The  Similarity  of  Brothers  and  Sisters  in  Mental 
Traits.    Psychological  Review  1917,  Vol.  24,  pp.  235-238. 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  231 

Assuming  that  the  infection  does  not  depreciate  the  mental 
organism  structurally,  even  then  in  terms  of  attitude  there 
would  remain  the  question  of  functional  depreciation.  It  is 
desirable  then  to  devise  a  test  to  measure  the  attitude  of  the 
learner  toward  his  task.  Admittedly  this  is  difficult  of  meas- 
ure, yet,  the  difference  in  the  learning  gains  by  two  groups 
who  on  setting  out  upon  the  task  are  of  equal  intelligence, 
should  be  a  fairly  good  measure. 

Suppose,  for  example,  two  such  groups  are  tested  at  a  given 
time  and  after,  say  six  months,  are  retested,  and  show  the 
same  gains.  Even  then  their  learning  abilities  under  normal 
procedure  may  not  be  proved  equal ;  for  if  on  the  other  hand 
the  test  had  been  repeated  75  times  on  as  many  days  or  if  the 
children  had  been  at  regular  school  work  with  their  teacher, 
the  one  group  may  have  lagged  in  interest  far  more  than  the 
other. 

Certainly  the  factor  of  attitude  and  of  stimulation  under 
peculiar  conditions  have  not  been  carefully  taken  into  account 
in  some  of  the  widely  read  experiments  whose  data  seem  to 
show,  for  example,  that  rebreathed  but  circulated  air  is  quite 
as  good  for  the  child's  mental  function  as  freshened  air  or 
that  there  is  no  such  thing  as  mental  fatigue  or  that  certain 
drugs  have  little  or  no  impairing  effect  on  mental  functioning, 
when  they  offer  no  such  conclusive  proof  at  all. 

Professor  James,  somewhere,  has  pointed  out  that  few  per- 
sons work  up  to  the  limit  of  their  capacity;  J.  J.  B.  Morgan,21 
that  subjects  lifting  weight  not  visible,  by  means  of  pulleys, 
tend  to  lift  with  the  same  speed,  within  certain  limits,  regard- 
less of  the  intensities  of  the  weights.  It  has  been  shown22 
that  learners  continue  to  improve  under  intensive  practice  to 
a  very  remarkable  degree  under  stimulation  of  rivalry.  In 
another  study23  the  writer  has  shown  that  by  learning  against 
time,  the  ordinary  learning  record  can  be  improved  to  sur- 
prising degree. 

A  learning  test,  then,  over  a  long  period  of  time,  with  many 
practices,  on  members  of  a  large  hookworm  group  compared 
with  similar  data  on  a  large  non-hookworm  group,  both  of 
practically  the  same  mental  and  social  status  at  the  beginning 
of  the  test,  with  the  diagnosis  and  cure  of  the  disease  under 
control,  should  yield  a  definite  answer  to  the  question  of  the 
mental  influence  of  hook  worm  disease. 

21  J.  J.  B.  Morgan,  The  Overcoming  of  Distraction  and  other  Re- 
sistances.   Archives  of  Psychology  #35,  Science  Press. 

22  Garry  C.  Myers,  Hazel  Coburn,  Helen  Collins,  School  and  Society 
1918,  Vol.  8  pp.  597-600. 

23  Garry  C.   Myers,   Learning   Against   Time.    Jr.   Ed.    Psychology, 
February  1915,  pp.  115-116. 


232  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 


PLATE  1 


PLATE  2 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  233 


PLATE  3 


PLATE  4 


234  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 


PLATE  5 


PLATE  6 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  235 


PLATE  7 


PLATE  8 


236  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 


PLATE  9 


PLATE  10 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  237 


PLATE  11 


PLATE  13 


23$  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 


PLATE  13 


PLATE  14 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  239 


PLATE  15 


PLATE  16 


240  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 


PLATE  17 

SUCCESSIVE    TEN    PER    CENT   GROUPS 

Highest  2nd     3rd     4th      5th      6th     7th     8th 


10%     10%   10%   10%   10%   10% 


9th  Lowest 

10%   10%  10%     10% 


PLATE  18 

SUCCESSIVE   TEN    PER    CENT    GROUPS 

Highest  2nd     3rd     4th      5th      6th     7th     8th     9th  Lowest 
10%     10%  10%   10%   10%   10%   10%   10%  10%     10% 


INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM  24 1 


PLATE  19 

SUCCESSIVE   TEN    PER    CENT   GROUPS 

Highest  2nd     3rd     4th      5th      6th     7th     8th     9th  Lowest 

10%     10%  10%   10%   10%   10%   10%   10%  10%     10% 


COLORED    TROOPS 


l\  HOOKWO" 

9 \  NON-HOOt 


-  162    CASES 
ZQ2   CASES 


PLATE  20 

SUCCESSIVE    TEN    PER    CENT    GROUPS 

Highest  2nd     3rd     4th      Sth      6th     7th     8th     9th  Lowest 
10%     10%   10%   10%   10%   10%   10%  10%  10%     10% 


242  INTELLIGENCE  OF  TROOPS  INFECTED  WITH  HOOKWORM 


COLORED    TROOPS 


BETA 

HOOKWORM  — 

NON-HOOKWORM 


290  CASES 
-3O6CASES 


PLATE  21 

SUCCESSIVE    TEN    PER    CENT    GROUPS 

Highest  2nd     3rd     4th      5th      6th     7th     8th     9th  Lowest 
10%     10%   10%    10%   10%   10%   10%   10%   10%     10% 


COM  MON     SCALE 


HOOKWORM  6I2CASES 

NON-HOOKWORM 612  CASES 


PLATE  22 

SUCCESSIVE    TEN    PER    CENT    GROUPS 

Highest  2nd     3rd     4th      5th      6th     7th     8th     9th  Lowest 
10%     10%   10%   10%   10%   10%   10%   10%   10%     10% 


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